How can a Normal Map affect the look of a model?
Normal maps simply trick the render engine into thinking that the face is at a differnt angle at that place. It cannot control height (It can still appear to create hight difference, but the are not accurate*) only angle.

Limitations

- Changing the silhouette: normal maps won't change the outline



Another angle




- Accurate depiction of height




Working With Limitations:


Design your mesh to work with these limitations


make sure the silhouettes match
make sure all height differences are clearly define. 

Adding enough supporting geometry to complete the illusion